(1) Field of the Invention
The present invention relates to a process and an apparatus for producing insulators. More particlarly, the invention relates to a process and an apparatus for producing insulators each having a rubbery housing integrally formed around an outer periphery of a core.
(2) Related Art Statement
In order to increase an insulating distance along a surface of the insulator, techniques are known to provide a number of shade portions around cores. As a process for forming a housing of rubber around an outer periphery of a core in the production of the insulator, a transfer molding process and an injection molding process have been employed.
Producing methods based on the transfer mold process are disclosed in Japanese patent application Laid-open Nos. 63-128,916 and 2-248,225. A molding apparatus used in the transfer molding process is constituted as shown in FIG. 1. A cavity 100 is defined between an upper mold unit 101 and a lower mold unit 102 when the mold units are closed. The cavity 100 has an inner peripheral surface for forming a sheath portion and shade portions of a housing of an insulator. A heater is provided in or for each of the upper and lower mold units 101 and 102. Injection gates 103 are provided in the upper mold unit 101 or the lower mold unit 102 (in this case, the gates 103 are provided in the lower mold units 101). A cylinder 104 is located under the lower mold unit 102, and is provided with a press-in piston 105. A pot 106 is defined between the piston 105 and the molds 101 and 102 inside the cylinder 104. Supports 108 are movable in a direction orthogonal to an axis of the cavity 100 by cylinders 109, and are adapted to support a core 107 in the cavity 100 on one side.
In the transfer molding process, the core 107 made of an FRP (fiber-reinforced resin) is first set inside the cavity 100 along the axis thereof. The core 107 is supported in this state by means of the supports 108. Then, a raw rubber composition is charged into the pot 106, and softened there by heating. Next, the softened rubber composition is fed into a space between the core 107 and the inner peripheral surface of the cavity 100 through the gates 103 by means of the piston 105. At that time, the core 107 is pressed by the raw rubber composition. However, since the core 107 is supported by the supports 108 on a side opposite to the gates 103, the core is not bent or displaced. The rubber composition is heated at a given temperature in the space defined by the inner periphery of the cavity 100 and the outer periphery of the core for a given time after the rubber composition is fully charged. Thereby, the raw rubber composition is vulcanized in this state to form a rubbery housing around the core 107.
However, according to the transfer molding process, the maximum possible length of the rubbery housing per one molding step is about 2 m considering that a long core 107 will be bent through forced charging of the raw rubber composition inside the cavity even when the supports 108 are provided. On the other hand, the length of the housing for the insulator currently used for super high voltage power transmission reaches as much as 5 m or more.
For this reason, since the length of the housing molded by the transfer molding is short, a plurality of the housings of the insulators must be integrally joined together when a long housing is necessary. Therefore, since a plurality of the housings must be molded separately, it unfavorably takes a long time to effect the molding for obtaining the necessary housing. In addition, when adhesion at the joined portion between the housings joined together is insufficient, water invades the insulator through the joined portion. Consequently, insulating property of the joined housings is deteriorated.
Furthermore, the supports 108 must be retracted from the inside of the cavity 100 at the final stage of filings the raw rubber-composition inside the cavity under pressure. Therefore, it is a troublesome operation to retract the supports 108. Moreover, since the molding used in the transfer molding process requires the pot, the gate, the supports, etc., the apparatus is complicated, and its production costs rise due to the complexity thereof.
On the other hand, Japanese patent application Laid-open No. 60-500,929 discloses an injection molding process for the production of the insulator. A molding apparatus used in this process is constructed as shown in FIG. 2. That is, a cavity 110 is defined between an upper mold unit 111 and a lower mold unit 112 united together through parting surfaces thereof when the mold units are closed, and an inner periphery of the cavity 110 has a shape constituting a barrel portion and shade portions of an insulator to be molded. An injection gate 113 is provided in one of the mold units (the mold unit 111 in this embodiment). A protective sleeve 114 is supported by supports 115 arranged in the upper and lower mold units 111 and 112. The supports 115 are movable by a cylinder 117 in a direction orthogonal to an axis of a core 116 placed inside the cavity.
The core 116 is first arranged inside the cavity 110, and the supports 115 are inwardly moved to press the protective sleeve 114 externally fitted around the core 116 at tips thereof. Thereby, the core 116 is held in the cavity 110 along an axis of the cavity. Next, a raw rubber composition is injected into the cavity around the outer periphery of the core 116 through the gate 113, so that a barrel portion and shelves of a housing are formed around the outer periphery of the core 116.
However, according to the injection molding process, the maximum possible length of the obtainable rubbery housing cannot be increased as in the case of the transfer molding process, because the core 116 is bent by the pressure of the rubber composition injected inside the cavity 110.
Therefore, when a long insulator is required, plural housings obtained in the injection molding process must be joined. Therefore, it takes a long time to mold a necessary number of the housings. In addition, since a joined portion exists in the long housing, insulating property of the housing is unfavorably deteriorated. Furthermore, there exists a troublesome operation to retract the supports 115 from the interior of the cavity 110 at a terminal stage in injecting the raw rubber composition. Further, the mold requires the supports, etc., which makes the mold complicated.